Apparatus and method for chemical mechanical polishing with improved uniformity

ABSTRACT

A chemical mechanical polishing (CMP) apparatus includes a workpiece carrier configured for retaining a workpiece thereupon, a polishing platen configured for retaining a polishing pad thereupon, and an electromagnetic coil surrounding a periphery of the workpiece carrier. The electromagnetic coil is configured to provide a magnetic field of alternating polarity to cause the rotation of ferromagnetic slurry particles disposed on the workpiece to facilitate polishing of the workpiece.

BACKGROUND

The present invention relates generally to semiconductor device processing techniques and, more particularly, to an apparatus and method for chemical mechanical polishing with improved uniformity.

Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is patterned by lithographic processes and subsequently etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate (i.e., the exposed surface of the substrate) becomes increasingly non-planar; that is, the topography of the exposed surface contains significant variations in the height of the surface. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process because the lithographic processes can handle only limited variations in the height of the surface. Therefore, there is a need to periodically planarize the substrate surface.

Chemical mechanical polishing (CMP) is the most commonly used process in semiconductor processing for reducing the aforementioned non-planar topography of the semiconductor surfaces. In a typical implementation of chemical mechanical polishing, the substrate is mounted on a carrier head or polishing head. A polishing pad and retaining ring, typically of a greater diameter than the wafer, are provided on the opposite side of the carrier. The polishing pad and wafer are pressed together while both the carrier head and the polishing pad are rotated. This is achieved by providing the carrier head with a controllable load (i.e., pressure) on the substrate to push it against the polishing pad. The carrier head and polishing pad may be rotated at different rates, and with different axes of rotation to help remove the material uniformly (that is, the average amount of removed material does not depend on the location within the substrate). The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A standard polishing pad has durable roughened surface, whereas a fixed-abrasive pad has abrasive, submicron particles (e.g., ceria (CeO₂)) embedded in a containment media. A polishing slurry, including at least one chemically reactive agent, and abrasive particles (where a standard pad is used) is supplied to the surface of the polishing pad.

With standard pad polishing, the passive motion of the slurry is caused by mechanical movement of the moving parts (e.g., wafer, carrier head, polishing pad, platen, etc.). However, the mechanical rotation of the moving parts is characterized by a non-uniform rate of movement of slurry particles in the radial direction. As such, controlling the rate of polishing becomes a challenge since the linear velocity of the slurry necessitates a compensation of the polishing rate with respect to other variables such as the down force of the polishing pad, and the angular velocity of the polishing pad and/or chuck. Any polishing mechanism that provides a higher polishing rate for higher linear velocity of the polishing slurry faces the challenge of converting uniform angular velocity of the carrier head and polishing pad into uniform linear velocity of the polishing slurries across the substrate. Accordingly, it would be desirable to be able to provide a CMP mechanism that provides a more uniform polishing rate across a semiconductor wafer.

SUMMARY

The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a chemical mechanical polishing (CMP) apparatus including a workpiece carrier configured for retaining a polishing pad thereupon, a polishing platen configured for retaining a workpiece thereupon, an electromagnetic coil configured to generate a magnetic filed on the workpiece, and an alternating current supply to provide alternating current through the electromagnetic coil.

In one embodiment, the electromagnetic coil surrounds the periphery of the workpiece carrier. The magnetic field is substantially perpendicular to the plane of the substrate.

In another embodiment, the electromagnetic coil is enclosed within the frame that also contains the polishing pad. The magnetic field is substantially perpendicular to the plane of the substrate.

In still another embodiment, the electromagnetic coil is embedded in an enclosure in which the polishing pad and polishing head are contained and either the enclosure or the assembly containing the polishing pad and polishing head slide in or out to facilitate the loading and unloading of the substrate. The magnetic field is substantially within the plane of the substrate.

TECHNICAL EFFECTS

As a result of the summarized invention, a solution is technically achieved in which a magnetic slurry is used in combination with an electromagnetic coil that provides a magnetic field of alternating polarity. The alternating magnetic field imparts a spin on the magnetic slurry particles, which in turn creates additional and more uniform pressure on a workpiece, thereby enhancing erosion of material during CMP.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:

FIG. 1( a) is a top view of a conventional CMP apparatus wherein the polishing pad and polishing head are depicted off-axis to illustrate a typical implementation;

FIG. 1( b) is a side view of the CMP apparatus of FIG. 1( a);

FIG. 2( a) is a schematic diagram of a CMP apparatus with magnetically spinning slurry particles responsive to an alternating magnetic field, in accordance with a first embodiment of the invention;

FIG. 2( b) is a side sectional view of the apparatus shown in FIG. 2( a);

FIG. 3( a) is a schematic diagram of a CMP apparatus with magnetically spinning slurry particles responsive to an alternating magnetic field, in accordance with a second embodiment of the invention;

FIG. 3( b) is a side sectional view of the apparatus shown in FIG. 3( a);

FIG. 4( a) is a schematic diagram of a CMP apparatus with magnetically spinning slurry particles responsive to an alternating magnetic field, in accordance with a third embodiment of the invention; and

FIG. 4( b) is a side sectional view of the apparatus shown in FIG. 4( a).

DETAILED DESCRIPTION

Disclosed herein is an apparatus and method for implementing chemical mechanical polishing with improved uniformity. Briefly stated, a magnetic slurry is used in combination with an electromagnetic coil that provides a magnetic field of alternating polarity. The alternating magnetic field imparts a spin on the magnetic slurry particles, which in turn creates additional (and more uniform) pressure on the wafer, thereby enhancing erosion of material. Consequently, large wafers (e.g., 300 mm or more) may be polished with a high degree of uniformity, with the polishing rate less dependent upon the radial distance from the center of the chuck/pad. Furthermore, the need for radial adjustment of down force is minimized, and polishing can be carried out at reduced rotational speeds of the polishing head and/or pad.

FIG. 1( a) and FIG. 1( b) illustrate a conventional CMP apparatus 10, in which a carrier head 12 is shown to be carrying a wafer (or more generally a) substrate 14 (FIG. 1( b)). The surface of the substrate 14 to be polished faces downward toward a polishing pad 16 and the slurry 18 disposed thereon. A table 20 (holding pad 16) and a corresponding shaft 22 for the rotation for the table 20 are also depicted in FIG. 1( b). A shaft 24 for rotation of the carrier head 12 is shown to be off-axis from with respect to the axis of rotation of the table 20.

Referring now to both FIGS. 2( a) and 2(b), there is shown a schematic diagram of a CMP apparatus 100 with magnetically spinning slurry particles responsive to an alternating magnetic field, in accordance with a first embodiment of the invention. As is shown, the apparatus 100 includes a wafer carrier (chuck) 102 having a semiconductor wafer (or more generally, a workpiece) 104 held thereon. In addition, a CMP polishing pad 106 is attached to a polishing platen (head, table) 108, configured for rotational motion by means of a shaft 110. The rotating pad 106 is brought into contact with the wafer 104, which is supplied with magnetic slurry particles 112 thereon. The slurry particles 112 may include, for example, a silica or cerium based material along with a ferromagnetic material as well. Alternatively, the slurry may be manufactured by coating silica or cerium based material on spherically shaped ferromagnetic material. Optionally, the carrier 102 (and thus the wafer 104) may also be independently rotated, in the opposite direction, with respect to the pad 106 and polishing head 108, as indicated by the shaft 114 shown in dashed lines.

Although FIGS. 2( a) and 2(b) depict a CMP apparatus with concentric shafts for the wafer carrier and for the polishing platen, it is not necessary for both centers to coincide. Nor is it necessary to limit the size of the polishing platen to that of the carrier head. In fact, size of the polishing platen may be bigger than that of the carrier head and the two axes of rotation may be off-centered as depicted in FIG. 1( a) and FIG. 1( b). This provides the benefits of a more uniform polishing rate provided by the conventional (nonmagnetic) component of polishing rate while not compromising the benefits of the magnetic component of the polishing rate.

As further shown in FIGS. 2( a) and 2(b), the apparatus 100 further includes a coil 118 wound around the periphery of the wafer 104, chuck 102 and magnetic slurry particles 112. The coil 118 has an alternating current (AC) excitation source 120 so as to provide a magnetic field of varying polarity. In the exemplary embodiment depicted, the coil 118 is disposed within a housing 122 that is structurally independent from the rest of the CMP apparatus 100 (i.e., the chuck 102 and polishing head 108), the footprint of which is generally indicated as 124 in FIG. 2( a). However, as described herein after, other embodiments contemplate the coil 118 also being incorporated into the chuck 102 and/or polishing head 108 as well.

In operation, the ferromagnetic slurry particles 112 (in the presence of an applied magnetic field of alternating polarity through coil 118) change their orientation so as to align their internal magnetization with the externally applied magnetic field. Coupled with a small amount of mechanical rotation (through the rotating pad 106 and/or chuck 102), the magnetic slurry particles 112 rotate between the pad 106 and the wafer 104. Because the rotation of the particles 112 is substantially uniform along the surface of the wafer 104, the resulting amount of energy, force and pressure applied to the wafer from 104 the spinning slurry particles 112 is also substantially uniform, thus leading to more uniform polishing.

The magnetic potential energy, E, of a solid particle having a magnetization, M, and a volume, V, is given by the expression:

E=(−V)M·B  (eq. 1)

Wherein B represents the magnetic flux density of an external applied magnetic field applied to the particle.

Therefore, if the magnetic particle does not move when the polarity of the externally applied magnetic field switches from B to −B, then the energy difference, ΔE, between the two states becomes:

ΔE=2VMB  (eq. 2)

Because staying in the same magnetic orientation as an external magnetic field changes direction is not an energetically “favorable” condition, the particle will, as a result, turn (i.e., spin) around in accordance with the least perturbation to the direction of the particle.

The volume of an individual slurry particle, having a radius, R, of about 1 micron is given as follows:

$\begin{matrix} {\quad\begin{matrix} {V = {{4/3}\mspace{11mu} \pi \; R^{3}}} \\ {= {{4/3}\mspace{11mu} {\pi \left( {10^{- 6}m} \right)}^{3}}} \\ {= {{4/3}\mspace{11mu} \pi \; 10^{- 18}m^{3}}} \end{matrix}} & \left( {{eq}.\mspace{14mu} 3} \right) \end{matrix}$

The term “remanence” refers to the residual magnetism left within a medium after an external magnetic field has been removed. A typical value of remanence for a ferromagnetic material is on the order of about M≈10,000 gauss (G)=1 tesla (T).

The magnetic flux density, B, generated by an electromagnetic coil is given by:

B≈μNI/L  (eq. 4)

Wherein μ is the permeability of free space (air), N is the number of turns of wire around the electromagnet (e.g., 100,000), I is the current through the coil (e.g., 10 amperes) and L is the length of the magnetic circuit (e.g., 1 meter).

Applying these exemplary values for the coil 118 to equation 4 above, the generated magnetic flux density is approximately:

4π10⁻⁷m⁻¹(100,000)(10A)/(1 m)=4π10⁻¹T≈1T

For a magnetic force confined within a high permeability material, an order of magnitude estimation of force is given by:

$\begin{matrix} {\quad\begin{matrix} {F \approx {{E/2}\; R}} \\ {= {{VMB}/R}} \\ {\approx \left( {{4/3}\mspace{11mu} \pi \; 10^{- 18}{m^{3}\left( {1T} \right)}{\left( {1T} \right)/10^{- 6}}m} \right.} \\ {= {{4/3}\mspace{11mu} \pi \; 10^{- 12}N}} \end{matrix}} & \left( {{eq}.\mspace{14mu} 5} \right) \end{matrix}$

Converting the above to an estimation of force per unit area (pressure), P, on the wafer yields:

$\begin{matrix} {\quad\begin{matrix} {P \approx {F/R^{2}}} \\ {= {{VMB}/R}} \\ {= {\left( {{4/3}\mspace{11mu} \pi \; 10^{- 12}N} \right)/\left( {10^{- 6}m} \right)^{2}}} \\ {\approx {4\; {Pa}}} \end{matrix}} & \left( {{eq}.\mspace{14mu} 6} \right) \end{matrix}$

It can thus be seen from the above calculations that the pressure magnetically generated on the substrate is comparable with the downward pressure of conventional CMP processing techniques. Although the polishing pad and the application of some downward pressure is still used to contain the magnetic slurry between the wafer and the pad (and to enable contact between the slurry and wafer), the slurry itself can generate about the same or even more pressure on the wafer for enhanced erosion of material through its magnetic coupling with the external magnetic field.

As stated above, the electromagnetic coil 118 (in addition to being located within a structurally isolated housing) could also being incorporated into the chuck 108 and/or polishing platen 102 as well. For example, FIGS. 3( a) and 3(b) illustrate a second embodiment of a CMP apparatus 200 with magnetically spinning slurry particles responsive to an alternating magnetic field. As particularly shown in FIG. 3( b), the polishing platen 202 has a coil housing 222 integrated into a non-rotating portion therein. Thus, where the apparatus 200 provides for both a rotating polishing pad and a rotating carrier head, the coil housing 222 may be mechanically isolated from a rotating portion of the polishing platen, as indicated by dashed line 214. The top view of FIG. 3( a) depicts the relationship between the coil housing 222 of the chuck 202 with respect to the footprint of the carrier head 102, generally indicated as 224. One skilled in the art will appreciate that a variant of the second embodiment may be easily constructed from FIGS. 3( a) and 3(b) where the coil housing is integrated in a similar manner into the carrier head 102 instead of the integrating it into the polishing platen as depicted in FIGS. 3( a) and 3(b).

While not explicitly shown in the figures, one skilled in the art may easily construct another variant version of the second embodiment wherein the radius of the polishing platen is greater than the radius of the carrier head, and the two shafts for the rotation of the polishing platen and carrier head are off axis as described in FIG. 1( a) and FIG. 1( b).

Finally, FIGS. 4( a) and 4(b) illustrate a third embodiment of a CMP apparatus 300 with magnetically spinning slurry particles responsive to an alternating magnetic field. Whereas the longitudinal axis of the coil 118 is along the axis of rotation of the apparatus in FIGS. 2( a), 2(b), 3(a) and 3(b), the longitudinal axis of the coil 302 is essentially orthogonal to the axis of rotation of the apparatus 300. As further shown in FIGS. 4( a) and 4(b), the coil is wound around the entire CMP apparatus, which is enclosed within a housing 304. The slurry particles 112 are still caused to spin in accordance with an alternating magnetic field, but the field in this embodiment alternates in and out of the page from the perspective of FIG. 4( b), as opposed to in and out of the page from the perspective of either FIG. 2( a) or FIG. 3( a). In other words, the direction of the magnetic field is essentially perpendicular to the surface of the substrate to be polished in the first and second embodiments while the direction of the magnetic field is essentially within the plane defined by the hypothetical, ideally polished substrate. The footprint of the polishing head and chuck with respect to the housing 304 is indicated at 306 in FIG. 4( a).

As with the first and second embodiments, one skilled in the art can easily construct another variant version of the third embodiment wherein the radius of the polishing platen is greater than the radius of the carrier head and the two shafts for the rotation of the polishing platen and carrier head are off axis as described in FIG. 1( a) and FIG. 1( b).

While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A chemical mechanical polishing (CMP) apparatus, comprising: a workpiece carrier configured for retaining a workpiece thereupon; a polishing platen configured for retaining a polishing pad thereupon; and an electromagnetic coil surrounding a periphery of the workpiece carrier, said electromagnetic coil configured to provide a magnetic field of alternating polarity to ferromagnetic slurry particles disposed on the workpiece.
 2. The CMP apparatus of claim 1, wherein a longitudinal axis of the electromagnetic coil is perpendicular to a plane defined by the surface of contact between the polishing platen and the workpiece carrier.
 3. The CMP apparatus of claim 2, wherein at least one of the polishing platen and the workpiece carrier has a shaft connected to the center thereof and is configured for rotational motion around the shaft.
 4. The CMP apparatus of claim 3, wherein the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are coaxial.
 5. The CMP apparatus of claim 3, wherein the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are not coaxial and the diameter of the polishing platen is larger than the diameter of the workpiece carrier.
 6. The CMP apparatus of claim 2, further comprising a housing containing the electromagnetic coil, wherein the housing comprises a non-rotating part of the polishing platen.
 7. The CMP apparatus of claim 6, wherein at least one of the polishing platen and the workpiece carrier has a shaft connected to the center thereof and is configured for rotational motion around the shaft.
 8. The CMP apparatus of claim 7, wherein the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are coaxial.
 9. The CMP apparatus of claim 7, wherein the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are not coaxial and the diameter of the polishing platen is larger than the diameter of the workpiece camer.
 10. The CMP apparatus of claim 2, further comprising a housing containing the electromagnetic coil, wherein the housing comprises a non-rotating part of the workpiece carrier.
 11. The CMP apparatus of claim 10, wherein at least one of the polishing platen and the workpiece carrier has a shaft connected to the center thereof and is configured for rotational motion around the shaft.
 12. The CMP apparatus of claim 11, wherein the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are coaxial.
 13. The CMP apparatus of claim 11, where the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are not coaxial and the diameter of the polishing platen is larger than the diameter of the workpiece carrier.
 14. The CMP apparatus of claim 1, wherein a longitudinal axis of the electromagnetic coil is within a plane defined by a surface of contact between the polishing platen and the workpiece carrier.
 15. The CMP apparatus of claim 14, wherein the electromagnetic coil is within a housing enclosing the polishing platen and the workpiece carrier.
 16. The CMP apparatus of claim 14, wherein at least one of the polishing platen and the workpiece carrier has a shaft connected to the center thereof and is configured for rotational motion around the shaft.
 17. The CMP apparatus of claim 16, where the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are coaxial.
 18. The CMP apparatus of claim 16, where the shaft of rotation for the polishing platen and the shaft of rotation for the workpiece carrier are not coaxial and the diameter of the polishing platen is larger than the diameter of the workpiece carrier.
 19. A method for planarizing a workpiece, the method comprising: disposing a plurality of ferromagnetic slurry particles upon the polishing platen; affixing the workpiece upon a workpiece carrier; placing the workpiece carrier upon the polishing platen such that the ferromagnetic slurry particles contact both the polishing platen on one side and the workpiece on the other side; and, subjecting the ferromagnetic slurry particles to a magnetic field of alternating polarity so as to cause the rotation of the ferromagnetic slurry particles. 